A method and a system for actuating a brake of a vehicle that can be electromechanically operated by means of an actuator that consists of an electric motor and a gearbox connected downstream of the electric motor are known from EP 1 154 922 B1, wherein the electric motor comprises a design-related revolution rate-torque characteristic curve, wherein the gradient of the revolution rate-torque characteristic curve of the electric motor is altered according to the desired operating force as well as the gradient of the operating force by reducing components of the electromagnetic field of the electric motor so that a higher revolution rate is achieved for the same torque. With the brake described, the frictional lining is fed against the brake disk electromechanically.
A generic hydraulic brake system is described in DE 10 2010 040 097 Al. In this case, a hydraulic cylinder-piston arrangement is used as a pressure source, in which a piston is driven by an electric motor with the interposition of a rotation-translation gearbox. The pressure source is connected to wheel brakes by means of hydraulic lines and inserted hydraulic valves.
Electronically commutated permanently energized synchronous machines, also known as brushless motors, comprise a stator with at least two, in particular three, phase windings and a rotor with at least one pole pair that is disposed perpendicular to the axis of rotation and that is formed by one or more permanent magnets disposed in or on the rotor. If one or more phase windings is energized, the rotor aligns itself in the existing magnetic field. For a specific actuation, the rotor position must be determined, which is carried out for example by means of a resolver or rotary encoder.
The regulation of the phase currents is often carried out in a coordinate system that is fixed relative to the rotor, wherein a d-axis in the direction of the rotor magnet field and a q-axis at an angle of 90° thereto (electrical angle, combined with the mechanical angle by means of the pole-pair number) are considered. A current flowing in the q-axis direction determines the delivered torque (in a motor without a reluctance torque) and is therefore referred to as a torque-forming current (iq). Below a revolution rate limit, the field attenuation current (id) flowing in the d-axis direction is kept at zero to maximize efficiency. The coordinate system that is fixed relative to the rotor rotates in the opposite direction to the stator, therefore the phase currents or voltages to be applied are determined by means of a suitable transformation using the rotor position.
With increasing revolution rate, an ever larger counter voltage is induced in the phase windings, so that the achievable revolution rate is limited by the available supply voltage. There is therefore a natural voltage limit for the revolution rate, which is achieved when energizing the phase windings exclusively with a torque-forming current at full actuation, i.e. without reducing the voltage by means of pulse width modulation. By applying a suitable current in the direction of the negative d-axis, i.e. in a field attenuation mode, higher revolution rates can be achieved.
A device for operating a synchronous machine with a stator with which three winding phases are associated and a rotor is known from DE 102007033145 A1. The device is designed to determine a target value of a magnet field-forming current in a coordinate system rotating with the rotor of the synchronous machine depending on a magnet field-forming raw target current component of the coordinate system circulating with the rotor and a magnet field-forming current limit of the coordinate system circulating with the rotor, and to do so such that the target value of the magnetic field-forming current is limited to the magnetic field-forming current limit, wherein the magnitude of the magnetic field-forming current limit lies below a typical tip-over limit for the synchronous machine in the field attenuation mode. Furthermore, the device is designed to reduce the magnitude of a torque-forming current limit of the coordinate system circulating with the rotor in the region of the limit of the magnet field-forming current depending on the magnet field-forming raw target current component and the magnet field-forming current limit.
A field attenuation regulator of this type enables a voltage reserve to be held at all working points of the motor for imposing new target current values. However, holding a voltage reserve always has the effect of always imposing a larger field attenuation current id than is necessary for reaching the working point. This increases the Ohmic losses of the motor and thus degrades the efficiency of the drive. It is moreover a disadvantage that the field attenuation regulator requires a certain time to adjust the necessary magnetic field attenuation current, which results in extending the control time for dynamically position-regulated systems.
For example, in automobile applications electric motors are operated up to 1 kW from a low supply voltage (for example 12V), so that the Ohmic resistance of the phase windings relative to the inductive reactance can no longer be neglected. In general, instabilities in the regulation or actuation of the motor can occur at high revolution rates of the rotor by an interaction between magnetic field attenuation and torque-forming currents.
The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.